varying vec3 vertex_light_position;varying vec3 vertex_normal;void main(){    // Set the diffuse value (darkness). This is done with a dot product between the normal and the light    // and the maths behind it is explained in the maths section of the site.    float diffuse_value = max(dot(vertex_normal, vertex_light_position), 0.0);       // Set the output color of our current pixel    gl_FragColor = gl_Color * diffuse_value;}